You won’t lose all of your strength and endurance (thank goodness!), but taking a break will chip away at any improvements you’ve made in the weeks prior. After that, you’ll lose another 50 percent of what fitness gains remain with each missed week. “It's all about supply and demand,” says Jason Karp, Ph.D., exercise physiologist and author of Running for Women. “When we exercise, we stimulate the synthesis of proteins, like mitochondria and enzymes, to meet the demand we place on our bodies. When we stop exercising, we eliminate the demand, so we start to lose our supply.”

Why does your body turn on you so fast?It's a chain reaction. First, the amount of blood available for your heart to pump will start to decline after just a week. The volume of mitochondria in your muscles also decreases when you go cold turkey. “These are the small power plants that produce all of our aerobic energy,” Magill explains. And you’ll lose capillary density (that’s the amount of small blood vessels that carry oxygen and nutrients to your cells). At the same time, your nervous system stops using the pathways that control muscle contractions, causing muscle weakness and less power behind each movement. This will also lead to a less efficient fuel or exercise economy, which means your body will need more oxygen to get the job done. Plus, the enzymes responsible for metabolism in your muscles decline. It all adds up to one thing: You won’t be able to push your heart, lungs, muscles, and mind as hard as you once could.

Freaking out about your past two weeks off?We know: It sounds scary. But keep in mind it’s also good to take a break from time to time. “Reducing your fitness once or twice a year—for two to three weeks at a time—allows your nervous system to recover, muscles and connective tissue to fully repair, and other systems to recuperate from the demands of training,” Magill explains.

It’s the reason endurance athletes in running, cycling, swimming, and other sports build downtime into their training schedules. Overtraining can actually be worse than detraining because it can lead to injury or burnout. Wondering about tapering before a big event? A few days of downtime actually leaves you in peak shape: Your body has had a chance to recover and repair from your last tough workouts, but hasn’t yet lost any fitness. (When is it okay? 9 Reasons to Skip Your Workout... Sometimes.)

“Unplanned breaks, on the other hand, can leave you sucking wind in the middle of a training cycle,” Magill cautions. Whenever possible, consciously plan breaks after a period of hard training by reducing your activity, but not going cold turkey altogether. It will be easier on your body than complete inactivity. Then ease back into a new training routine when you’re refreshed (typically about two to three weeks, most experts agree). (Start again the right way, with how to Jump Back into Your Fitness Routine.)

Want to stay in shape during planned downtime or an unexpectedly busy schedule?Intensity is more important than duration or frequency to maintain fitness, so at the very least, do a few intense workouts rather than skipping the gym completely, Karp suggests. Magill recommends exercising at least three times a week at the same intensity as usual, but cutting the time for each sweat session by half (or even two thirds), but at the same intensity as your usual workouts. For example, if you normally ride the elliptical for 60 minutes at a 9-minute per mile pace, ride for 30 minutes at that same pace to maintain your fitness.

If you fall off the wagon completely, don’t worry.You can get it back with time. But you’ll have to be patient: “Unfortunately, it takes a lot longer to get fitness back than it does to lose it because it takes longer to synthesize proteins than for those proteins to be degraded,” Karp says. (Jump back in safely with How to Get Back to Working Out.)

If you lose endurance fitness—those mitochondria and capillaries—you’ll need the same amount of time to rebuild as it took you to gain initially (about 12 to 14 weeks to reach peak shape, Magill says). (Speed up progress in 4 Weeks to Fit: Total-Body Makeover.)

Now for some good news: “If you’ve lost neuromuscular fitness—those pathways that control your muscle function—you can sometimes rewire your body in as little as a day,” Magill encourages. “Short hill sprints are great for this if you’re a runner!”